1.1 Technical Field
The invention relates to powder self-adhesive compositions with improved resistance to creep, based on thermoplastic and/or heat-curable resins for coating metal substrates.
2.1 Description of Related Art
Polyamide, polyolefin, polyester and/or polyurethane resins are commonly employed for coating metal substrates, especially because of their good mechanical properties such as abrasion resistance and impact strength and their chemical inertness towards many products such as hydrocarbons, bases and inorganic acids.
However, it is known that the adhesiveness of these resins to metals is generally inadequate because, for example, of poor wettability. To overcome this defect, the metal support is coated with an undercoat, called adhesion primer, to ensure the bonding and the mechanical anchoring of the polyamide powder. In general, the adhesion primer employed is based on heat-curable resins and is applied in the form of powder, in solution or in suspension in organic or aqueous solvents. Provision must therefore be made for additional facilities for possible removal of the solvents and for curing the primer before the coating of the substrate thus clad with the polyamide powder. In addition, curing and/or drying the primer significantly increases the duration of the coating operations, and hence their cost.
Given the above-mentioned disadvantages, it is increasingly being preferred to eliminate the adhesion primer and to improve the direct adhesion of the coating to the substrate.
Thus, in European Patent Application No. EP 412 888 there is described mixtures of polyamides and of epoxy/sulfonamide resins which can be employed for coating metal substrates without employing any adhesion primer. These mixtures of polyamide and of epoxy/sulfonamide resins in powder form can be applied onto the substrate with an electrostatic gun. It then suffices to introduce the substrate thus clad into an oven to obtain melting of the powder and a uniform coating. The substrate can also be preheated above the melting point of the powder and immersed in a fluidized bed of the powder.
PCT International Patent Application No. PCT/FR95/01740 discloses pulverulent compositions based on polyamide, on ethylene copolymers and on unsaturated carboxylic acids or their vinyl or other esters, and, optionally, on copolymer of ethylene and vinyl alcohol. However, the adhesion of such existing coatings which are applied without a primer is still inferior to that of the coatings with a primer, especially where resistance to creep is concerned.